1. Field of the Invention
The present invention relates to a plasma processing method and plasma processing device preferable for carrying out processes using plasma such as etching to materials such as silicon oxide, silicon nitride, low dielectric constant film (low-k film), polysilicon and aluminum in a process for manufacturing semiconductor devices.
2. Description of the Related Art
In the manufacture of semiconductor devices, plasma processing devices are widely used to carry out processes such as film deposition and etching. Such plasma processing devices are required to realize a highly accurate processing performance and mass-productivity in correspondence to finer processing of devices. Now, a large problem in mass production is the deterioration of yield caused by foreign particles adhering to the wafer during plasma processing.
Foreign particles adhering to the wafer during plasma processing may cause crucial defects of the device such as disconnection of wires or short circuit. Moreover, as the device becomes more miniaturized, even the very minute foreign particles which had not been an issue in the past may have a greater influence. Though it is possible to remove the foreign particles through wet processing after the plasma processing, it is not desirable since the increased processes raise the manufacturing costs of the devices. Thus, particular attention is paid in carrying out plasma processes to reduce the amount of foreign particles being generated, to eliminate the generated particles and to prevent the particles from falling on the wafer.
Japanese Patent Application Laid-Open Publication No. 11-162946 (patent document 1) discloses an example of an art for eliminating foreign particles during plasma processing. The publication of patent document 1 discloses generating lines of magnetic force B that are diverged upwards, and having foreign particles move out of range of the area above the semiconductor wafer along the lines of magnetic force B.
Japanese Patent Application Laid-Open Publication No. 5-47712 (patent document 2) discloses another example of an art for eliminating foreign particles during plasma processing. Patent document 2 discloses reducing the amount of foreign particles by providing a second plasma generating electrode on the circumference of a lower electrode, and applying a high frequency voltage to the second plasma generating electrode immediately before suspending the plasma discharge in order to produce a high density sub-plasma on the circumference of the lower electrode and forming the distribution of sub-potential in a processing chamber to push out the foreign particles which are negatively charged and held up in the vicinity of the main surface of a semiconductor wafer.
Further, it has been known widely that the foreign particles in the plasma fall on the wafer not when the plasma processing is carried out but when the plasma is turned on and off. For example, “H. H. Hwang, Appl. Phys. Lett. 68, p. 3716, 1996” (non-patent document 1) discloses that during plasma processing, that is, during the time in which RF bias is applied to a wafer, the foreign particles are trapped in the boundary between a sheath formed directly above the wafer and bulk plasma, so that they are prevented from falling on the wafer.
On the other hand, “Journal of Applied Physics 97, 043306, 2005” (non-patent document 2) discloses an expression related to the sheath thickness ds of the RF sheath formed directly above the wafer when RF bias is applied to the wafer in plasma. Further, “Clean technology, January 2004, p. 9” (non-patent document 3) refers to a force Fg applied to a static foreign particle from the gas flow surrounding the particle.
The art disclosed in patent document 1 utilizes the fact that foreign particles in the plasma are electrically charged. In general, when foreign particles enter the bulk plasma, the particles are negatively charged since the diffusion coefficient of electrons is much greater than the diffusion coefficient of positive ions.
As known, the charges in motion in a magnetic field are subjected to Lorentz force from the magnetic field and move in such a manner as to coil around the magnetic field, so the direction of movement of the charges are bound by the magnetic lines of force. If the mass is small as in the case of electrons (or more accurately, if the specific charge e/m is large, wherein e represents quantity of electric charge and m represents mass) the motion thereof can be sufficiently bound by the magnetic field of a few Gauss to a few hundred Gauss used in plasma processing devices. However, if the mass is close to that of ions (which is a few thousand times greater than that of electrons), it is impossible to bound the action thereof by a magnetic field of a few Gauss to a few hundred Gauss.
For example, if a magnetic field of 75 Gauss is applied to a plasma used generally for plasma processing, the Larmor radius of electrons is 1 mm or smaller, whereas the Larmor radius of ions is approximately 20 to 30 mm, which is one digit greater than the mean free path of gases (which is approximately a few mm). This means that electrons are capable of revolving for a few times around the lines of magnetic force before colliding against gas molecules, or in other words, the motion of electrons can be bound by magnetic fields, whereas ions collide against gas molecules before revolving around the lines of magnetic force and the direction of motion thereof is changed, or in other words, the motion of ions cannot be bound by magnetic fields. Further, even a particle having a diameter as small as 0.1 μm has a mass greater by approximately 8 digits than that of ions, it is impossible to bound the motion thereof by magnetic fields even if the particles are charged. Moreover, the mass of foreign particles are proportional to the third power of the particle radius, whereas the quantity of charge of the foreign particles is proportional to the square of the surface area of the particles or square of the radius, so the specific charge e/m reduces as the particle diameter increases. In other words, it is practically impossible to eliminate foreign particles from the range of the wafer by the method disclosed in patent document 1.
Moreover, the art disclosed in patent document 2 is not realistic from the viewpoint of practical application. It has a large drawback in that the arrangement of the device becomes complex by installing a second plasma generating electrode for generating a high density sub-plasma on the circumference of the lower electrode and a power supply for applying power to the electrode, and the related costs are greatly increased thereby. Moreover, the electrode for generating the second plasma is consumed, possibly becoming the source of foreign particles and contaminants. Even further, the effect of reducing foreign particles which is the initial object of the art falls short of expectations.
At first, since the sub-plasma is generated on the circumference of the lower electrode at which the plasma generating unit and the wafer are not directly opposed, the sub-potential is not sufficiently formed above the wafer from which the foreign particles must be eliminated. In other words, even if a sub-plasma is generated according to the art, its influence does not reach the area above the wafer. This becomes more significant in a high-pressure region in which the diffusion velocity of plasma is low. On the other hand, even if the pressure is reduced and the diffusion velocity of plasma is increased, the fast diffusion speed causes the sub-plasma density distribution above the wafer to become more uniform, by which the desired sub-potential cannot be formed sufficiently. Therefore, it is questionable that the art disclosed in patent document 2 exerts any effect of eliminating foreign particles.